Various types of radiation detectors are used, including powder phosphor screens or needle image plates (needle IP), direct radiography detectors (amorphous silicon, amorphous selenium, Cmos, phosphor detector arranged for direct radiography etc.) or the like.
A radiation image is recorded on such a detector (also called ‘plate’) by exposing it to an x-ray field. The radiation image that is temporarily stored by the detector is read out in a read out system (also called ‘digitizer’) where the exposed detector is scanned with light of an appropriate wavelength and where the image-wise modulated light emitted by the detector upon stimulation is detected and converted into a digital image signal representative of the radiation image.
An embodiment of a digitizer is described in Ref. 1 (U.S. Pat. No. 6,369,402 B1).
The scanning technique in the digitizer could be flying-spot or one line at a time. See Ref. 5 (R. Schaetzing, R. Fasbender, P. Kersten, “New high-speed scanning technique for Computed Radiography”, Proc. SPIE 4682, pp. 511-520).
A phosphor screen or needle image plate is commonly conveyed in a cassette and is not part of the read out system.
The signal to noise ratio (SNR) or normalized noise power spectrum ((N)NPS) of the image data must be analyzed in order to evaluate the diagnostic capacity of the radiographic system. This implies that for each detector the uniformity of the detector needs to be evaluated, and corrected for. The techniques to do so are known in the state of the art.
The SNR or (N)NPS of the digitizer must be determined for different uniform dose levels to be able to study its behavior over the dynamic range.
Instead of using different detectors for each dose setting, a phantom target is used that contains a number of sub-targets each with a known absorption level for x-ray exposure. See Ref. 2 to 4 (EP 02 100 669.7, EP 02 100 792.7, EP 05 106 112.5).
Exposure of the detector then gives an image that contains the raw data needed for SNR or (N)NPS calculation. Every sub-target contains a region of interest (roi) with a known and constant attenuation and is exposed to a uniform radiation field.